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Some 500 tenants and shoppers fled to safety on Monday morning when the TechnoMart shopping mall in Seoul began to tremble for unknown reasons. The Gwangjin District office ordered all tenants to stay away for three days and began a precision safety check. The mall accommodates about 1,200 small electronic appliance shops, a large shopping center, a multiplex cinema, and restaurants.

Tremors

"The building rocked up and down right after 10 o'clock in the morning and I felt dizzy, so I rushed to the elevator with about 60 colleagues," said a 35-year-old woman who gave her surname as Lee and works on the 20th floor. The water in plastic bottles and leaves of potted plants shook visibly, she added.

A staffer of Prime Center, the property firm managing the mall, said, "Mild tremors occurred, but we regard them as temporary since there were no signs of further tremors."

The 39-story building complex was built based on an anti-earthquake design so that strong winds can shake the building from side to side.

According to tenants, tremors were felt mainly on floors 18 to 39. But some people on the seventh and ninth floors also said they felt tremors. "I thought it was an earthquake," said an office worker on the 22nd floor who gave his surname as Choi. "At the time, people who were walking fast or talking on the phone didn't feel the tremor, but nearly everyone who was sitting down did."

Structural Faults?

Experts did not rule out that the tremors could be harbingers of disaster. Lee Won-ho, a vice president of the Architectural Institute of Korea, said, "Tremors should have been felt on all floors of the building if there was a problem with the foundation of the building. So it's probably not a problem with the foundation if tremors were felt only on some floors." But he said the tremors could be the result of a new load condition that was not reflected in the original design being created as a result of modification in the use of the building.

Chun Young-chul, a member of the board of directors at the Korea Institute of Registered Architects, said, "If tremors occurred only on certain floors, there are two probabilities, structural defects or partial impact." He said there may be defects in the joints on the floor where the tremors originated, or a minor gas explosion in the building shook the entire building briefly.

"There must be signs like tremors or noise, because no reinforced steel structure simply collapses," Chun said. "It's highly likely that the latest tremors are a sign of imminent collapse or accident, if there was a noise or a crack in the structure before."

Safety Checks

This building was built in 1998 and designed to withstand quakes of a magnitude of 7.0 or higher, according to Prime Center. The building has twice been rated safe. It passed a visual safety check by an inspection firm in March. Visual safety checks, where experts check for any cracks in columns, walls and beams, are carried out twice a year.

And according to the Gwangjin District office, it got a Grade B in a precision safety check in 2008. Grades A to C mean that there is no problem with safety. Grade D is given to buildings that need repairs or reinforcement before they can be used. Grade E means the building is so unsafe that occupants should evacuate immediately.

Park Jong-yong of the Gwangjin District office said, "First of all, experts are conducting a visual safety check. We'll decide when tenants can go back to minimize loss for traders."